A shellfish creeper? (10)
I believe the answer is:
periwinkle
This might be a double definition.
'a shellfish' is the first definition.
(I know that periwinkle is a type of sea snail)
'creeper?' is the second definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for periwinkle that I've seen before include "Plant with five-petalled flowers" , "gastropod" , "Ornamental plant" , "Plant; mollusc" , "Evergreen creeper" .)